Diazepam potentiates postsynaptic inhibition in bulbar respiratory neurons of cats

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Role of α1- and α2-GABA(A) receptors in mediating the respiratory changes associated with benzodiazepine sedation

BACKGROUND AND PURPOSE

The molecular substrates underlying the respiratory changes associated with benzodiazepine sedation are unknown. We examined the effects of different doses of diazepam and alprazolam on resting breathing in wild-type (WT) mice and clarified the contribution of α1- and α2-GABA(A) receptors, which mediate the sedative and muscle relaxant action of diazepam, respectively, to these drug effects using point-mutated mice possessing either α1H101R- or α2H101R-GABA(A) receptors insensitive to benzodiazepine.

EXPERIMENTAL APPROACH

Room air breathing was monitored using whole-body plethysmography. Different groups of WT mice were injected i.p. with diazepam (1-100 mg·kg(-1) ), alprazolam (0.3, 1 or 3 mg·kg(-1) ) or vehicle. α1H101R and α2H101R mice received 1 or 10 mg·kg(-1) diazepam or 0.3 or 3 mg·kg(-1) alprazolam. Respiratory frequency, tidal volume, time of expiration and time of inspiration before and 20 min after drug injection were analysed.

KEY RESULTS

Diazepam (10 mg·kg(-1) ) decreased the time of expiration, thereby increasing the resting respiratory frequency, in WT and α2H101R mice, but not in α1H101R mice. The time of inspiration was shortened in WT and α1H101R mice, but not in α2H101R mice. Alprazolam (1-3 mg·kg(-1) ) stimulated the respiratory frequency by shortening expiration and inspiration duration in WT mice. This tachypnoeic effect was partially conserved in α1H101R mice while absent in α2H101R mice.

CONCLUSIONS AND IMPLICATIONS

These results identify a specific role for α1-GABA(A) receptors and α2-GABA(A) receptors in mediating the shortening by benzodiazepines of the expiratory and inspiratory phase of resting breathing respectively.

A model of the respiratory central pattern generator

We have developed a model of the mammalian respiratory central pattern generator (rCPG) to mimic the salient characteristics of its constituent medullary neurons. This model was designed as a network of Hodgkin-Huxley type medullary neurons under the hypothesis that synaptic and network effects predominate over ionic influences in determining the pattern of firing seen in individual neurons. After obtaining satisfactory mimicry of these patterns we validated the model to a different set of data in order to examine its robustness in the face of transient perturbations.

Multiple centrally acting antidotes protect against severe organophosphate toxicity

BACKGROUND

Accumulation of acetylcholine in the central nervous system is believed to account for the rapid lethality of organophosphate pesticides and chemical nerve agents. Diazepam is known to supplement atropine therapy, but its specific mechanism of action is uncertain.

OBJECTIVES

To test four centrally acting agents for early antidotal efficacy in severe dichlorvos poisoning in the murine model.

METHODS

The up-and-down method was used to dose four candidate antidotes: diazepam, xylazine, morphine, and ketamine. Antidotes were administered subcutaneously to unsedated adult Sprague-Dawley rats who were pretreated with 3 mg/kg intraperitoneal glycopyrrolate. All animals received 20 mg/kg of dichlorvos subcutaneously 5 minutes later. A blinded observer adjudicated the outcomes of 10-minute mortality and survival time.

RESULTS

All animals pretreated with either no antidote (8/8 deaths) or glycopyrrolate alone (8/8) died within 10 minutes of dichlorvos injection. Pretreatment with diazepam (3/9 deaths), or xylazine (3/9), decreased lethality substantially (Fisher p = 0.007; median effective doses, 0.12 mg/kg and 3.0 mg/kg, respectively). Intermediate doses of morphine (3.1 to 5.5 mg/kg) resulted in survival, but higher doses did not, presumably because of excessive respiratory depression (7/11 deaths; p = 0.09). Ketamine (7/8 deaths) was ineffective as an antidote. Survival times also were prolonged in the diazepam and xylazine groups (log-rank p < 0.001) and, to a lesser degree, the morphine group (p = 0.07).

CONCLUSIONS

Doses of diazepam, xylazine, and morphine below those used for deep sedation protect against severe dichlorvos poisoning, implying that several distinct central mechanisms are each sufficient to avert lethality. These findings suggest new possibilities for prophylaxis or therapy.

Glutamic acid decarboxylase-immunoreactivity of bulbar respiratory neurons identified by intracellular recording and labeling in rats

To distinguish the GABAergic neuron in the ventral respiratory group (VRG) of rats, immunohistochemical staining of glutamic acid decarboxylase (GAD) was performed in neurons that had been individually identified by in vivo intracellular recording and labeling with neurobiotin. A total of five types of respiratory neurons were identified and labeled; augmenting inspiratory (aug-I, n=12), decrementing or early inspiratory (early-I, n=3), inspiration-expiration phase spanning or late inspiratory (late-I, n=3), decrementing expiratory or postinspiratory (PI, n=8), and augmenting or stage 2 expiratory (E2, n=3). In addition, expiration-inspiration phase-spanning or pre-inspiratory neurons (pre-I, n=2) were recorded, but not labeled. The membrane potential trajectory of each neuron type resembled that previously described in cat, suggesting a comparable neuronal organization between the two species. According to the axonal arborization, those labeled neurons were further classified as propriobulbar (6 aug-I, all early-I, all late-I, and 3 PI), bulbospinal (2 aug-I and all E2) and cranial-motor neurons (4 aug-I and 5 PI). GAD-immunoreactivity was consistently detected in the propriobulbar neurons, while it was not seen in cranial-motor and bulbospinal neurons. In addition, GAD-immunoreactive varicosities were found surrounding the somatic and dendritic surface of all labeled neurons. The present results illustrate that the propriobulbar types of early-I, aug-I, late-I and PI neurons are GABAergic inhibitory neurons and virtually all types of respiratory neurons receive GABAergic inputs in the rat's VRG.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

GABA(A) receptor-mediated inspiratory termination evoked by vagal stimulation in decerebrate cats

To identify the GABAergic inhibitory mechanisms involved in inspiratory termination or off-switching (IOS), the effects of a specific enhancer of GABA(A) receptors, midazolam, and an antagonist, bicuculline, on vagally evoked inspiratory inhibitions and IOS were investigated in decerebrate cats. Stimulation of vagal afferents at late inspiration provoked either reversible inspiratory inhibition or IOS, depending on the stimulus intensity. Each response occurred at a constant latency (phase 1). The reversible response was triphasic, consisting of an early (phase 2) inhibition, a brief (phase 3) excitation and a late (phase 4) inhibition in the phrenic neurogram, and early (phase 2) IPSPs, brief (phase 3) EPSPs and late (phase 4) IPSPs in bulbar inspiratory (I) neurones. With an increasing stimulus intensity, phase 4 inhibitions were increased in amplitude and duration, leading to IOS. Midazolam (0.1 mg/kg i.v.) increased more selectively phase 4 IPSPs than phase 2 IPSPs in I neurones, and decreased the threshold for evoking IOS by producing an earlier and larger phase 4 IPSPs. Bicuculline (1.0 mg/kg i.v.) had an opposite effect. These results suggest that the late inhibitory response evoked by vagal stimulation in the I neuronal pool organizes an initial phase of IOS which is mediated by GABA(A) receptors.

Sedative and ventilatory effects of midazolam infusion: effect of flumazenil reversal

The purpose of this study was to evaluate the effects of flumazenil (1 mg i.v.) on the ventilatory response of premedicated patients receiving a continuous infusion of midazolam for sedation. After assessing baseline ventilatory function using a modified Read rebreathing method for determining hypercapnic ventilatory drive, 16 healthy outpatients were administered fentanyl, 50 micrograms i.v., and midazolam 2 mg i.v., followed by a variable-rate midazolam infusion, 0.3-0.5 mg.min-1. Upon termination of the midazolam infusion, serum midazolam concentrations were measured and ventilatory function was reassessed. Then, 10 ml either saline or flumazenil (1 mg) were administered according to a randomized, double-blind protocol. Ventilatory function was subsequently measured at 5 min, 30 min and 60 min intervals after study drug. Compared with the baseline value, midazolam infusion reduced tidal volume and increased respiratory rate and alveolar dead space. However, midazolam did not decrease the slope of the CO2-response curve. Flumazenil reduced the degree of midazolam-induced sedation and the decrease in tidal volume (P < 0.05), but not the change in resting respiratory rate. In some patients, the ventilatory response to hypercarbia actually decreased after flumazenil administration compared with the immediate prereversal (sedated) values. It is concluded that midazolam infusion, 0.43 mg.min-1, did not impair CO2-responsiveness. Flumazenil's effect on central ventilatory drive was more variable than its reversal of midazolam-induced sedation.

Endogenous benzodiazepine system and regulation of respiration in the cat

Benzodiazepines, a class of drugs widely used as anxiolytics, can induce a depression of respiration. This study was designed to determine if endogenous benzodiazepine ligands could act in a similar fashion and exert a tonic inhibitory influence on respiration. Administration of a benzodiazepine antagonist should then facilitate respiration. This might be especially visible in hypoxia, the condition characterized by both central respiratory depression and potentially enhanced benzodiazepine expression. We addressed this issue by comparing the effects on the phrenic neurogram of the specific benzodiazepine antagonist flumazenil (200 micrograms i.v. boluses) in the contrasting conditions of hypoxia and hyperoxia in anesthetized, both spontaneously breathing and paralyzed ventilated cats. Contrary to our hypothesis, flumazenil showed a modest but definite inhibitory effect on respiration. Flumazenil also lengthened the duration of the Hering-Breuer inspiratory inhibition. The respiratory depression was neither related to chemical drive nor to the GABA receptor complex, for it was sustained after antagonism of GABA with picrotoxin and bicuculline. We conclude that the endogenous benzodiazepine system is unlikely to play an inhibitory role in the regulation of respiration. The physiologic role of this system remains to be established.

The role of inhibitory amino acids in control of respiratory motor output in an arterially perfused rat

The respiratory effects of drugs affecting GABAergic and glycinergic transmission were examined in order to assess the role of synaptic inhibition in breathing rhythmogenesis. Experiments were performed in the arterially perfused in situ brainstem-spinal cord preparation from adult rats (Hayashi et al., 1991, J. Neurosci. Meth. 36:63-70). Administration to the perfusate of agonists of GABAA, GABAB, and glycine receptors reduced both the frequency and amplitude of the activity recorded from the phrenic and hypoglossal nerves. Similar effects were observed following the infusion of aminooxyacetic acid (a blocker of GABA-transaminase). Picrotoxin (0.1-2 microM), bicuculline (0.05-0.2 microM), strychnine (0.1-1 microM) and phaclofen (0.1-0.2 mM) usually increased the frequency and amplitude of inspiratory bursts. Perfusion with low Cl- (8 mM) solution elicited tonic discharge followed by reversible arrest of the respiratory activity. It is concluded that synaptic inhibition is involved in the respiratory rhythm generation process in the mature mammalian brain. As data from the literature indicate that interference with central inhibitory processes does not largely affect the rhythm generation process in newborn rats, a possibility is discussed that the brainstem respiratory generator undergoes a developmental change from a 'pacemaker' driven circuit at the neonatal stage to a network requiring post-synaptic inhibition in the mature brain.

GABA-immunoreactive boutons make synapses with inspiratory neurons of the dorsal respiratory group

Intracellular labelling with horseradish peroxidase (HRP) combined with gamma-aminobutyric (GABA) immunocytochemistry was used to assess the GABAergic input to inspiratory bulbospinal neurons of the dorsal respiratory group in the cat. The relationship between GABA-immunoreactive (GABA-IR) boutons and intracellularly labelled neurons was examined at the light microscopic and ultrastructural levels. At the light microscopic level, GABA-IR boutons were frequently found in close apposition to dendrites and cell bodies of labelled neurons. The presence of synapses was confirmed with electron microscopy. In addition, synaptic specializations were observed between immunoreactive boutons and unlabelled terminals which in turn formed synaptic contacts with HRP-labelled dendrites, a finding consistent with presynaptic inhibition. These results demonstrate a direct GABAergic input to a functionally defined population of medullary respiratory neurons, and suggest involvement of this neurotransmitter in the control of these neurons.

Microiontophoresis of flurazepam on inspiratory and postinspiratory neurons in the ventrolateral medulla of cats: an intracellular study in vivo

Effects of flurazepam on the periodic inhibitory postsynaptic potentials (IPSPs) and on the action of locally applied gamma-aminobutyric acid (GABA) were studied in bulbar respiratory neurons of decerebrate cats using concentric multibarrelled electrodes for intracellular recording and extracellular iontophoresis. Iontophoresis of flurazepam augmented spontaneous IPSPs and increased the hyperpolarization induced by GABA. Iontophoretic application of bicuculline suppressed the action of flurazepam. The reversal potential for spontaneous IPSPs was similar to that for the GABA-response. Intracellular Cl- injection shifted both the IPSP wave and the GABA response in a depolarizing direction. Flurazepam enhanced these depolarizing responses. These results suggest that GABA mediates the postsynaptic inhibition in bulbar respiratory neurons.